Known projection display devices include front projection display devices in which a projection lens is arranged on the same side as an audience viewing light beams containing information reflected from a reflective display screen, and rear projection display devices in which a transmissive display screen is arranged between an audience and a projection lens that transmits light beams containing information to the transmissive display screen. In rear projection display devices, such as rear projection televisions, it is well known to arrange the transmissive display screen in the front of a cabinet that contains a light source and the projection lens facing the screen.
In such cabinets, a smaller cabinet is preferred. Therefore, there is a demand that the volume of the cabinet be decreased. For this reason, shortening the projection distance, i.e., the distance from the surface of a projection lens toward the projection screen to the projection screen enables making the television thinner but requires widening the projection field angle of the projection lens. These considerations have led to projection lenses with widened projection field angles, as set forth, for example, in Japanese Laid-Open Patent Applications 2001-337267, 2002-031754, 2002-365541, and 2003-195164 and Japanese Patent No.3487468.
Proposals for deflecting the optical path in projection lens systems have also been made for minimizing the height of the cabinet. Deflecting the optical path at an acute angle, as small an angle as possible, is desirable for miniaturizing devices in this way. Japanese Laid-Open Patent Application 2003-156683 discloses a mirror used to deflect the optical path and Japanese Laid-Open Patent Application 2003-248169 discloses a prism used to deflect the optical path.
In a rear projection display device, a correction of distortion among aberrations is often particularly important. Recently, a display method known as multi-display, in which multiple projection optical units are arrayed up and down and left to right in order to display a large picture has been frequently adopted. However, a projection lens for high correction of distortion is particularly required so that the seams of each unit are not strikingly evident in a large picture, which requires distortion to be kept within 0.3%. Generally, the front and rear lens groups on opposite sides of a stop in a wide-angle projection lens are highly asymmetrical, which makes distortion correction particularly difficult.
As a projection lens for projection display devices using liquid crystals, providing telecentric properties with regard to the illumination system is also important in order to prevent the occurrence of color irregularity and loss of light.
As described above, many projection lenses for miniaturizing a cabinet projection display device have been proposed, but simultaneously satisfying all of the various demands described above is very difficult. A projection lens that achieves the desired results with optical path deflection as desired, as well as satisfying demands related to projection field angle, distortion correction, f-number, and other requirements has not yet been realized.
The projection lenses described in Japanese Laid-Open Patent Applications 2001-337267, 2002-031754, 2002-365541, and 2003-195164 and Japanese Patent No. 3487468 do not have the required space for arranging an optical path deflector in order to achieve miniaturization. The projection lens described in Japanese Laid-Open Patent Application 2001-337267 has an insufficient field angle and an insufficient back focus, and also has a large number of lens elements. The projection lens described in Japanese Laid-Open Patent Application 2002-031754 does not provide a small enough f-number. The projection lens described in Japanese Laid-Open Patent Application 2003-195164 has insufficient field angles and does not provide a small enough f-number.
The projection lens described in Japanese Laid-Open Patent Application 2003-156683 is suitable for miniaturization because it provides a wide field angle and the optical path is deflectable at an acute angle in the lens system, but the use of the lens is limited because the back focus is short and distortion is great.
The projection lens described in Japanese Laid-Open Patent Application 2003-248169 is suitable for miniaturization because a prism is arranged to deflect the optical path in the lens system, but this is less favorable than a mirror in terms of weight and cost. According to this construction, a small f-number is not provided. Moreover, the space necessary for the arrangement of a mirror is larger than for a prism. Therefore, a mirror cannot simply replace the prism. Although a mirror provides advantages of less weight and lower cost than a prism, a larger space must be provided, which increases the difficulty of optical design as compared to using a prism.